The present invention relates to an inkjet recording device, and more particularly to a high-speed inkjet recording device including a refresh ink ejection device capable of performing ink refresh operations without stopping recording operations.
There have been proposed line-scan inkjet recording devices that print on a continuous recording sheet at high speed. This type of recording device includes a linear recording head that extends across the entire width of the continuous recording sheet. The recording head is formed with nozzles aligned in a row for ejecting ink. Each nozzle includes an ink chamber having an orifice and an energy generating member, such as a piezoelectric element or a heat generating element. When a driving voltage is applied to the energy generating member with the recording head facing the continuous recording sheet, then a pressure is applied to ink inside the ink chamber, whereby an ink droplet is ejected through the orifice. The ejected ink droplet impinges on the continuous recording sheet and thus forms a recording dot thereon. It is possible to control the ink droplet to impinge on a selected location on the continuous recording sheet based on a recording signal. The continuous recording sheet is rapidly transported in its longitudinal direction. By controlling both the sheet feedings and impact positions of ink droplets, recording dots are formed on scanning lines defined on the recording sheet, whereby a desired image is formed.
Various types of line-scan inkjet recording devices have been proposed, such as those using a continuous inkjet recording head and those using a drop-on-demand inkjet recording head. Although the drop-on-demand inkjet recording head has a slower printing speed than the continuous inkjet recording head, the drop-on-demand inkjet recording head has a simple ink system, and so is well suited for general-purpose high-speed recording device.
The present inventors have proposed an ink-droplet deflection type inkjet recording device that includes a charging/deflecting electrode in addition to the drop-on-demand type line-scan inkjet recording head. The charging/deflecting electrode is for deflecting ejected ink droplets so that the ink droplets impinge on desired locations on a recording sheet. The charging/deflecting electrode is disposed in confrontation with orifices to extend along a nozzle row. With this configuration, it is possible to control a plurality of ink droplets ejected from adjacent plural nozzles to impinge on a single pixel location in an overlapping manner. Therefore, even if one or more of nozzles become defective, it is possible to form recording dots using remaining nozzles. That is, the problem of missing information due to defective nozzles can be prevented. Moreover, unevenness in color density of resultant images due to unevenness in nozzle characteristics can be avoided, thereby enhancing reliability in printing operations.
Here, because the drop-on-demand type line-scan inkjet recording head ejects ink droplets in accordance with print data only when needed to form recording dots, some nozzles may not eject ink droplets for a relatively long period of time even during printing operations. If nozzles do not eject ink droplets for a certain time duration, ink clinging around corresponding orifices will dry out, preventing stable ink ejection. In order to overcome this problem, there has been proposed an inkjet recording device including a refresh ink ejection device.
A conventional refresh ink ejection device performs an ink refresh operation by stopping recording operations. Usually, the refresh ink ejection device moves a recording head to a predetermined refreshing position and then control the recording head to eject refresh ink droplets toward an ink receiving member. Japanese Patent Application-Publication No. 2000-211159 discloses a refresh ink ejection device that moves an ink receiving member to a location between a recording head and a recording sheet without moving the recording head. Because there is no need to move the recording head to a refreshing position, an ink refresh operation takes less time duration. A refresh ink ejection device disclosed in Japanese Patent Application-Publication No. HEI-11-334106 performs ink refresh operations without moving a recording head nor an ink collection member. The ink receiving member is disposed between the recording head and a recording sheet at a location away from an ink ejection direction. Ejected refresh ink droplets are deflected toward the ink receiving member by an air current or an electrostatic force and are collected by the ink receiving member without impinging on the recording sheet.
However, no matter which type of the above refresh ink ejection device an inkjet recording device includes, ink refresh operations require to stop printing operations with a resultant reduction in overall printing speed. Also, even when only a small number of refresh ink droplets are required to be ejected for maintaining a good ink ejection performance, excessive refresh ink droplets are inevitably ejected for mechanical reasons, so that ink is wasted more than necessary. Further, it has been difficult to use the conventional refresh ink ejection devices in the above-described high-speed line scan inkjet recording device that prints on a continuous recording sheet using a recording head having a width of the recording sheet for following reasons.
That is, ink ejection frequency greatly differs among nozzles of the recording head having such a head. For example, nozzles located near side edges of the sheet eject inks least frequently. However, even these nozzles need to reliably eject ink in a stable manner when required. Accordingly, it is necessary to perform the ink refresh operations at timings suitable for these nozzles. As a result, the recording operations are disrupted frequently and actual printing speed is greatly decreased.
Further, it is difficult to precisely and quickly stop and restart transporting a continuous recording sheet that is being transported rapidly, and so is difficult to print a high-quality image in succession.
Moreover, in order to dispose the ink receiving member between the recording head and the recording sheet, it is necessary to widen a gap between the recording head and the recording sheet. However, widening the gap between the recording head and the recording sheet degrades printing quality. It is also necessary to provide a sufficient gap between the ink receiving member and the recording sheet. Otherwise, the ink receiving member will be an obstacle to transport the recording sheet, causing paper jam. These problems of recording-quality degradation and paper jam are serious particularly in a device that collects refresh ink droplets by deflecting the same, because in this type of device, it is necessary, for deflecting the refresh ink droplets by a sufficient amount, to widen the gap or to shorten a distance between the ink receiving member and the recording sheet.
In view of forgoing, it is an object of the present invention to overcome the above problems and also to provide a refresh ink ejection device capable of ejecting necessary amount of refresh ink anytime when needed. It is also an object of the present invention to provide an inkjet recording device including a refresh ink ejection device that prevents degradation of printing quality and sheet jam by disposing an ink receiving member without widening a gap between a recording head and a recording sheet.
In order to overcome the above and other objects, a refresh ink ejection device according to the present invention comprises an ink ejection means for generating and ejecting an ink droplet, an ink collector for collecting the ink droplet, and a deflecting means for deflecting the ink droplet ejected from the ink ejection means so that the ink droplet impinges on the ink collector, and is characterized by that the deflecting means includes a conductive member having the same potential as ink that the ink ejection means holds, the conductive member being disposed in vicinity of where the ink droplet is generated, a back electrode disposed behind a recording medium; and a voltage application means for applying a voltage between the conductive member and the back electrode for generating a deflecting electric field, and that the ink droplet ejected from the ink ejection means is deflected by the deflecting electric field and then impinges on the ink collector, and that the ink collector is provided on a surface of the conductive member, the surface facing the recording medium.
With this configuration, the ink droplet ejected by the ink ejection means is deflected by the deflecting electric field generated by the voltage applying means and travels along a u-turn path toward the ink collector. Therefore, the ink droplet is collected by the ink collector without reaching the recording medium. Because the ink collector provided to the surface of the conductive member that is facing the recording medium, there is no need to increase a gap between the ink ejection element and the recording medium by a large amount in order to place the ink collector, preventing degrading recording precision and paper jam.
The deflecting electric field is preferably an angle deflecting electric field having a field element in a direction perpendicular to an ink ejection direction of the ink ejection means. With this configuration, the ink droplet is effectively deflected.
It is preferable that the conductive member have a protrusion protruding toward the back electrode. Alternatively the conductive member could include an orifice plate formed with a nozzle and a conductive plate provided on the orifice plate. With this configuration, the angled deflecting electric field can be easily generated. Also, the conductive member and the ink collector could be formed integrally with each other.
The ink collector could be an ink absorbing member. Alternatively, a narrow groove for leading ink could be formed in a surface of the ink collector. With this configuration, the ink droplet impinged on the ink collector is reliably collected.
The ink residing on the ink collector can be collected by using an ink receiving member that includes a vacuum pump or a large-capacity ink absorbing member connected to the ink collector.
Further, an inkjet recording device according to the present invention comprises an ink ejection means for generating and ejecting an ink droplet, a first control means for controlling the ink ejection means to eject an recording ink droplet, a second control means for controlling the ink ejection means to eject a refresh ink droplet, an ink collector for collecting the refresh ink droplet, and an deflecting means for deflecting the refresh ink droplet so that the refresh ink droplet impinges on the ink collector, and is characterized by that the deflecting means includes a conductive member having the same potential as ink that the ink ejection means holds, the conductive member being disposed in vicinity of where the ink droplet is generated; a back electrode disposed behind a recording medium; and a voltage applying means for applying a voltage between the conductive member and the back electrode so as to generate a deflecting electric field, and that the ink collector is disposed on a surface of the conductive member, the surface facing the recording medium, and that the deflecting electric field deflects the recording ink droplet as needed, wherein the deflected recording ink droplet impinges on the recording medium, and that the deflecting electric field deflects the refresh ink droplet so that the deflected refresh ink droplet impinges on the ink collector.
With this configuration, the recording ink droplet ejected by the first control means impinges on the recording medium and forms a recording dot. On the other hand, the refresh ink droplet ejected by the second control means is deflected by the angled deflecting electric field to flying along a U-turn path toward the ink collector, so that the refresh ink droplet is collected by the ink collector without reaching the recording medium. Because the ink is collected by the ink collector provided to the surface of the conductive member facing the recording medium, there is no need to increase a gap between the ink ejection means and the recording medium by a large amount, preventing degradation in recording precision and paper jam. Further, because the second control means can control to eject a selected number of refresh ink droplets at a selected timing, waste of ink is prevented, and the operation is performed flexibly.
The first control means can control to eject the recording ink droplet based on a recording signal during a recording operation, and the second deflecting means can control to eject the refresh ink droplet during a time interval where no recording ink droplet is ejected during the recording operation, without stopping the recording operation. Because there is no need to stop the recording operation for ejecting the refresh ink droplet, it is possible to avoid decrease in throughput. Further, because there is no need to stop and then restart transporting the recording medium, it is possible to form high-quality images continuously, so that the present invention is well adopted to a high-speed line-scanning inkjet recording device that forms images on a uncut-elongated recording medium.
It is preferable that the second control means control the ink ejection means to eject the refresh ink droplet at a lower ejection speed than the recording ink droplet. The refresh ink droplet ejected at a slow ejection speed is more easily deflected by the angled deflecting electric field and thus reliably collected by the ink collector.
It is preferable that the refresh ink droplet have a smaller weight than the recording ink droplet. The refresh ink droplet having a smaller weight is more easily deflected by the angled deflecting electric field and thus reliably collected by the ink collector.
It is preferable to provide a charging means for charging the refresh ink droplet and the recording ink droplet, the charging means charging the refresh ink droplet to a larger potential than the recording ink droplet. The refresh ink droplet charged with a greater potential is more easily deflected by the angled deflecting electric field and thus reliably collected by the ink collector.
The deflecting electric field is preferably an angled deflecting electric field having a field element in a direction perpendicular to an ink ejection direction of the ink ejection means. With this configuration, the ink droplet is effectively deflected.